The invention relates to a method of producing process steam from a black liquor derived in connection with the production of paper pulp.
In the production of chemical paper pulp it is conventional to recover the cooking chemicals to be used again. This is normally done by evaporating a spent cooking liquor, in the case of kraft cooking a so called black liquor, and directing the concentrated spent cooking liquor to some type of recovery boiler or gasification system. Before this, at least some of the heat content of the black liquor can be, and usually is, used, for example by expansion evaporation, i.e. flashing, in one or more expansion evaporation units. From the expansion evaporation unit exits black liquor of a higher concentration than the concentration of the incoming black liquor, and steam which can be used at a desired location in the production of paper pulp. Such steam is conventionally used for example to pretreat the cellulose containing raw material, e.g. the chips, in order to heat it and to drive out gases which are captured in cavities inside the chips. The steam however contains a considerable amount of non-condensable gases, such as hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide, and also for example methanol and turpentine. The mixture of these gases is liable to explosion, but as long as it is present together with steam there is no risk of explosion. However, when the steam, containing the just mentioned gases, is contacted with the cold chips, for example in a counter current manner in a chip bin, the steam condenses and the gases are accumulated at the top of the chip bin. In older systems, the gases have simply been let out into the air and have thus been diluted so that their concentration in the air has become lower than the lower explosion limit (LEL), whereby there has been no risk of explosion. Due to environmental restrictions, many pulp mills have however nowadays started to collect the gases in order to burn them at a concentration lower than the LEL. In the top of the chip bin for example, the concentration may however reach values above the LEL which has been known to cause explosion. Thus, there has arised a problem in the handling of the gases which are accumulated in the chip bin, or at any other location.
From SE-A-9703680-0, there is known a method of producing relatively clean steam with a relatively low content of non-condensable gases using heat from black liquor. The black liquor is, according to the method, not exposed to expansion evaporation, but the heat is instead used to indirectly heat a relatively clean liquid in a heat exchanger in order to vaporise it. The steam which is formed contains non or only small amounts of non-condensable gases and can be used to steam the chips without the risk of accumulation of explosive gases. There is also mentioned the possibility to introduce some black liquor into the heat exchanger to be vaporised, which is said to yield a steam which is not clean but which all the same has a lower concentration of non-condensable gases than steam from conventional expansion evaporation.
By the present invention, there is achieved a method of producing a process steam from black liquor, whereby the steam becomes essentially free from impurities such as non-condensable gases (e.g. hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide) and whereby recovery of turpentine is improved. The process steam is preferably used to pretreat cellulose containing raw material in the production of paper pulp.
This is achieved, according to the invention, by treating a first black liquor to give a second black liquor of a higher concentration than said first black liquor as well as a liquid of low concentration, whereafter said process steam is produced from said low concentration liquid. It is to be understood that the xe2x80x9chigher concentrationxe2x80x9d of the second black liquor refers primarily to its contents of organic compounds such as lignin, hemicelluloses etc, whereas the xe2x80x9clow concentrationxe2x80x9d of the liquid of low concentration refers primarily to its contents of non-condensable gases and turpentine, although of course the concentration of organic compounds also is low in this liquid.
According to one aspect of the invention, said first black liquor is evaporated in a first evaporation step to give a second black liquor of a higher concentration as well as a first steam, which first steam comprises non-condensable gases and turpentine, whereafter said first steam, in a subsequent step, is partly condensed in a first condensing step to give a first condensate, which first condensate in its turn is evaporated in a second evaporation step to give said process steam.
According to another aspect of the invention, a remaining gas from said first condensing step is at least partly condensed in a second condensing step, which second condensing step is performed at a slight vacuum, preferably at 0.70-0.99 bar (abs) and more preferred at 0.80-0.95 bar (abs), to give a turpentine containing second condensate and a gas phase comprising non-condensable gases. The turpentine containing second heat is instead used to indirectly heat a relatively clean liquid in a heat exchanger in order to vaporise it. The steam which is formed contains non or only small amounts of non-condensable gases and can be used to steam the chips without the risk of accumulation of explosive gases. There is also mentioned the possibility to introduce some black liquor into the heat exchanger to be vaporised, which is said to yield a steam which is not clean but which all the same has a lower concentration of non-condensable gases than steam from conventional expansion evaporation.
By the present invention, there is achieved a method of producing a process steam from black liquor, whereby the steam becomes essentially free from impurities such as non-condensable gases (e.g. hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide) and whereby recovery of turpentine is improved. The process steam is preferably used to pretreat cellulose containing raw material in the production of paper pulp.
This is achieved, according to the invention, by treating a first black liquor to give a second black liquor of a higher concentration than said first black liquor as well as a liquid of low concentration, whereafter said process steam is produced from said low concentration liquid. It is to be understood that the xe2x80x9chigher concentrationxe2x80x9d of the second black liquor refers primarily to its contents of organic compounds such as lignin, hemicelluloses etc, whereas the xe2x80x9clow concentrationxe2x80x9d of the liquid of low concentration refers primarily to its contents of non-condensable gases and turpentine, although of course the concentration of organic compounds also is low in this liquid.
According to one aspect of the invention, said first black liquor is evaporated in a first evaporation step to give a second black liquor of a higher concentration as well as a first steam, which first steam comprises non-condensable gases and turpentine, whereafter said first steam, in a subsequent step, is partly condensed in a first condensing step to give a first condensate, which first condensate in its turn is evaporated in a second evaporation step to give said process steam.
According to another aspect of the invention, a remaining gas from said first condensing step is at least partly condensed in a second condensing step, which second condensing step is performed at a slight vacuum, preferably at 0.70-0.99 bar (abs) and more preferred at 0.80-0.95 bar (abs), to give a turpentine containing second condensate and a gas phase comprising non-condensable gases. The turpentine containing second condensate is led to a turpentine decanter to be separated from the water. The gas phase is of low volume and high concentration (LVHC), which means that the concentration is above the upper explosion limit (UEL, limit normally at a concentration about 50-80%) and that the gas thus can be burned without risk of explosion.
It is a major advantage of the method according to the invention that heat from black liquor can be used in order to produce steam which is relatively free from turpentine and non-condensable gases. The black liquor is preferably expansion evaporated, as is conventional, but the flash steam is freed from the undesired gases before it is used in the process of paper pulp production, preferably by condensation and the reforming of steam from the condensate. An additional advantage is that turpentine recovery is improved by the method according to the invention. This also makes the method especially suited for softwood systems, due to softwood having a higher turpentine content than hardwood.
Additional aspects of the invention will be readily clear from the following detailed description and from the appending claims.